1. Field of the Invention
The present invention relates generally to a method of controlling an automotive charging system, and more particularly to a method of controlling current flow into an alternator field circuit and thereby controlling the electrical output of the alternator in response to impulse and/or transient electrical loads during idle speed conditions of a motor vehicle.
2. Discussion
It is becoming difficult to calibrate automatic idle speed (AIS) compensation for varying alternator loads, primarily due to ever increasing vehicle electrical loads that make it necessary to use larger amperage alternators (i.e., larger torque loading), larger intake manifolds creating a larger delay time in engine RPM response to AIS steps, smaller capacity batteries that have less capacity to handle current draws, and lower engine idle speeds resulting in alternator operation at less efficient speeds. Due to their inability to respond well to the periodic application of impulse electrical loads (e.g., the activation of the radiator fan), current methods for controlling the alternator field have been determined to be a primary cause of highly unacceptable engine speed fluctuations and other system instabilities at idle speed conditions.
Regulating the battery voltage level during vehicle operation consists of controlling the electrical current supplied to the alternator field circuit. This electrical current creates a magnetic field in the alternator that is proportional to the magnitude of the current. The magnitude of the magnetic field and the rotational speed of the alternator together determine the output of the alternator. The electrical field current is controlled and delivered by the powertrain control module (PCM). The PCM approximates a variable direct current by delivering a pulsewidth modulated control signal to the alternator field circuit. In a steady-state mode, alternator field control is accomplished by driving the alternator field control output pin of the microprocessor in the PCM to a prescribed ON/OFF pattern that effectively delivers one of several different duty-cycles (i.e., 0%, 12.5%, 25%, 50%, 75%, 87.5%, 100%). Periodically, for example once every 3.5 mS, the appropriate duty-cycle is chosen based on the magnitude of the difference between the most recent "trimmed" battery voltage level (from an AND conversion that occurs in the same routine once every 3.5 mS) and a desired operational battery voltage value, also referred to as the voltage regulator set point. However, missing from this control process is consideration of electrical load levels on the engine system at the time and also consideration of the engine speed at the time. As a result, the current control methods poorly manage the application of higher alternator field duty-cycles in response to impulse electrical loads (such as the activation of the radiator fan, the rear window heater, or the window motors), and it is this deficiency that has been determined to be the main cause of engine speed instability during idle speed conditions.
The problem that is caused by impulse or transient electrical loads is a result of how the system battery voltage drops (when they are applied) and how the control system detects and responds to these system battery voltage fluctuations. For instance, when the radiator fan is turned "on," the large in-rush of current that occurs because of the initial low impedance of the fan motor causes the system battery voltage to drop by a significant amount over a very short period of time. When the control system detects the system battery voltage drop, it attempts to recover by immediately increasing the alternator field current control duty cycle. This increase in alternator field current control duty cycle causes a proportional torque load on the engine as the alternator works to increase its output and restore the system battery voltage to its desired control value.
Current charging system control methods employ different strategies for managing the application of impulse loads and attempt to limit the corresponding maximum alternator field current control duty-cycle to avoid engine stall. These strategies produce undesirable engine speed fluctuations or undesirable dimming of headlamp and instrument panel lights. For example, when the control system determines that the radiator fan needs to be turned "on," it will open the AIS a prescribed number of steps and then delay a prescribed length of time before turning the radiator fan "on." This strategy effectively minimizes the possibility of the engine stalling when the radiator fan is turned "on" while operating at low engine speeds. It causes the engine speed to flare slightly before dropping sharply when the radiator fan is finally turned "on." It should be noted that this approach does not really make the idle speed any more stable, it merely makes the idle speed fluctuate above and below a target idle speed to prevent engine stall.
Therefore, it would be desirable to provide a method of controlling the alternator field current which would result in regulating the voltage in an automotive charging system in response to transient electrical loads. The method of the present invention will control current flow into an alternator field circuit during idle speed operating conditions, and thus regulate alternator output and engine torque loading to achieve an acceptable balance between engine speed fluctutation and system battery voltage drop. Upon detecting an impulse electrical load, the alternator field current flow will be gradually increased over a period of time based on the engine's rotational speed and also based on the magnitude of the delivered duty-cycle with respect to a maximum duty-cycle value. The new method effectively reduces engine speed and system voltage fluctuations that normally accompany the application of transient or impulse electrical loads. As these fluctuations are reduced, the noticeable side-effects are reduced, including changes in headlight, interior light and instrument panel light intensity, blower motor speed, and engine noise and vibration.